1. Field of the Invention
The present invention relates to a fluorescent substance usable in various light-emitting devices such as a white light-emitting diode (white LED) having a blue light-emitting diode (blue LED) or an ultraviolet light-emitting diode (UV LED), and a light-emitting device using the substance.
2. Description of the Related Art
Patent Document 1 discloses a white LED that combines a fluorescent substance with a semiconductor light-emitting device that emits visible light of shorter wavelengths in the blue to violet range, so as to mix the light emitted from the semiconductor light-emitting device with light wavelength-converted by the fluorescent substance, to result in a white light.
On the other hand, fluorescent substances using silicates, phosphates, aluminates and sulfates as the matrix material and transition metals or rare earth metals in the luminescent center are widely known.
With the increased output power of a white LED, the demand for heat resistance and durability of fluorescent substances has become greater. However, when using the conventionally known fluorescent substances indicated above, reduced luminosity of the fluorescent substances caused by use in higher temperature environments and degradation of the fluorescent substances due to prolonged exposure to blue light and UV excitation sources can result in reduced luminosity and color shifting in the white LED.
Recently, fluorescent substances consisting of nitrides and oxynitrides which have a stable crystal structure have been the object of much interest as fluorescent substances with little luminosity reduction with increased temperature and excellent durability.
Typical example of nitride or oxynitride fluorescent substances are sialons, which are solid solutions of silicon nitride. Like silicon nitride, sialons can have either a α-type or a β-type crystal structure. α-sialons which are containing with specific rare earth elements are known to have effective fluorescent properties, and their application to the white LED and the like has been considered (see Patent Documents 2-4 and Non-Patent Document 1).
On the other hand, β-sialons which are containing with rare earth elements have also been discovered to have similar fluorescent properties (see Patent Document 5).
β-sialons are solid solutions of β-silicon nitride, having the Si positions of the β-silicon nitride crystal substituted by Al, and the N positions substituted by O. Since there are two formula weights of atoms in a unit lattice, a general formula of Si6-zAlzOzN8-z is used. Here, z is 0-4.2, so the range of the solid solution is very broad, and the molar ratio of (Si, Al)/(N, O) must be maintained at 3/4. The crystal structure of β-type silicon nitride has P63 or P63/m symmetry, defined as a structure having ideal atomic positions (see Patent Document 5). Additionally, in general, a β-sialon can be obtained by adding, as raw materials, silicon oxide and aluminum nitride, or aluminum oxide and aluminum nitride, to silicon nitride, and heating.
By including Eu2+ in the crystal structure of β-sialon, it forms a fluorescent substance known as Eu-activated β-sialon that, upon excitation by UV to blue light, emits green light in the 520-550 nm wavelength range, which can be used as the green light component in a light-emitting device such as a white LED. Even among fluorescent substances that have been activated by Eu2+, Eu-activated β-sialons have an extremely sharp emission spectrum, and are fluorescent substances that are suitable for generating the green light components of backlight sources in liquid crystal display panels which require narrow bands of red, green and blue light.                Patent Document 1: Japanese Patent No. 2927279        Patent Document 2: Japanese Patent No. 3668770        Patent Document 3: Japanese Patent No. 3726131        Patent Document 4: Japanese Patent Application, Publication No. 2003-124527A        Patent Document 5: Japanese Patent Application, Publication No. 2005-255895A        Non-Patent Document 1: W. J. van Krebel, “On new rare-earth doped M-Si—Al—O—N materials”, TU Eindhoven, The Netherlands, pp. 145-161 (1998).        